This invention relates to a brushless motor and, more particularly, to a brushless D.C. motor of improved, simplified construction which is dynamically balanced and which, desirably, exhibits a low inertia.
In a typical brushless D.C. motor, a stator assembly is disposed in surrounding relationship to a rotor assembly. The rotor assembly generally is comprised of a rotor magnet having a desirable number of poles; and the stator assembly is formed of electromagnetic windings which are sequentially energized so as to impart a rotary motion to the rotor magnet. In a conventional motor of this design, the rotor magnet usually is a solid cylindrical magnet formed of, for example, a solid cylindrical ceramic ferrite magnet or arc-shaped sections, each section being formed of a ceramic ferrite magnet.
Other types of brushless D.C. motors are known in which the rotor magnet surrounds the stator assembly. This so-called outer-rotor D.C. motor also is comprised of relatively heavy cylindrical ceramic ferrite magnets or arc-shaped ceramic ferrite magnets. These ceramic ferrite magnets are fixed, usually by a suitable adhesive, to the inner surface of a cup-shaped rotor yoke. The rotor yoke, in this motor , is constructed of a material of relatively high magnetic permeability, such as soft steel. Typically, such a cup-shaped rotor yoke is made of soft steel plate. Consequently, because of this design, the rotor assembly is heavy and exhibits a relatively large moment of inertia This is particularly undesirable for those applications in which a motor having a small inertia is to be used.
Another disadvantage of brushless D.C. motors of the aforementioned type, that is, of the type in which the rotor magnet is formed either of a solid cylindrical ceramic ferrite magnet or arc-shaped ceramic magnets, is that various elements of the rotor assembly must be accurately machined so as to be within very small tolerances of a precise dimension in order to insure that the rotor assembly exhibits good rotary dynamic balance. This means that either the magnets, the rotor yoke or various rotary support elements must be manufactured within highly restrained tolerances. This, of course, results in high costs in manufacturing and assembling the brushless D.C. motor.
Further, if the brushless D.C. motor is the outer-rotor type in which the ferrite magnets of the rotor assembly must be secured to the inner surface of the cup-shaped rotor yoke, a relatively high assembly time is required for such a motor. Moreover, in a rotor assembly of this type, the rotational dynamic balance thereof may be readily disturbed. Consequently, after the rotor assembly has been assembled, balancing weights must be added thereto in order to return the assembly to its proper rotary dynamic balance.
Yet another disadvantage of brushless D.C. motors of the aforementioned type is that the overall motor construction becomes relatively complicated, and the size of the motor becomes unduly large, when additional devices are incorporated therein. For example, if a frequency generator, which normally is used to detect the rotary speed of the motor, is to be added, or if a transmission element, such as a pulley or the like, is to be added to the rotary shaft, still additional assembly time is required to manufacture such a motor. Also, the overall motor size and shape may become too large for a desired application, and the overall constructuion of the motor may be unnecessarily complicated.